Apparatus for generating and accelerating charged particles



1965 H. w. HENDEL ETAL 3,279,175

APPARATUS FOR GENERATING AND ACCELERATING CHARGED PARTICLES Filed Dec. 19, 1962 2 Sheets-Sheet 1 E: Em

MQQQQW WSW Gk NM A a m N United States Patent 3,279,175 APPARATUS FOR GENERATING AND ACCELER- ATING CHARGED PARTICLES Hans W. Hendel, Manasquan, and Tlieophile T. Reboul III, Princeton, N .J., assignors to Radio Corporation of America, a corporation of Delaware Filed Dec. 19, 1962, Ser. No. 245,804 14 Claims. (Cl. 60-202) This invention relates to methods of and apparatus for electromagnetically accelerating charged particles, and particularly for generating a plasma and for accelerating that plasma.

By a plasma is meant a gas including a mixture of positively and negatively charged particles, such as electrons and ions, in which gas the concentrations of posi tively and negatively charged particles are approximately equal.

The invention is especially useful for the propulsion of space vehicles. However, the invention may also be useful for studying thermonuclear reactions and aerodynamic effects.

Ion and plasma propulsion techniques have been proposed wherein ions or a plasma are produced and electrostatically and/or electromagnetically accelerated. Chief difficulties in ion acceleration are that (1) specific thrust (thrust per unit area of the cross-section of the beam in a plane perpendicular to the direction of the thrust) is limited by space charge effects, (2) the ion beam must be neutralized to maintain charge neutrality of the vehicle which is propelled by the beam, and (3) low efficiency at low specific impulse (velocity of the beam over the acceleration due to gravity). Various plasma accelerators have been suggested. These accelerators mainly use electrical pulses to drive the plasma and suffer from disadvantages of low duty cycle and produce low average specific thrust. Complex apparatus has been used in continuously operating plasma accelerators.

It is an object of the present invention to provide a method of and apparatus for plasma acceleration wherein the foregoing difficulties and disadvantages are eliminated.

It is a further object of the present invention to provide an improved method of and improved apparatus for generating and continuously accelerating a plasma.

It is still further object of the present invention to provide a method of and apparatus for plasma generation and acceleration which are less complicated and lower in cost than known plasma and ion acceleration schemes.

The foregoing objects and advantages may be obtained, according to the invention, in a plasma generator and accelerator wherein plasma is generated by introducing a neutral gas into a region where there are established combined magnetic and radio frequency electric fields, preferably at the electron cyclotron resonance frequency. The gas is ionized by the radio frequency electric field and a plasma is generated in the region. An externally generated plasma may be alternatively injected into this region. The combined fields selectively transfer radio frequency energy to the electrons of the plasma. The magnetic field extends beyond the first-mentioned region into a second region. The high energy electrons collide and expand in the direction of the magnetic field into this second region and are confined by the magnetic field. A space charge field forms which accelerates the ions in the plasma out of the first region and through the second region. The electrons thereby transfer their high energy to the ions and a high energy plasma beam of electrons and ions is expelled for propulsion or other purposes. Since the combined fields act primarily on the electrons, the magnetic field strength may be low as compared to the magnetic field strength used in ion devices and thus may 3,279,175 Patented Oct. 18, 1966 readily be generated and applied without difficulty. Ionic erosion also does not present a serious problem.

The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will become more readily apparent from a reading of the following description in connection with the accompanying drawings in which:

FIG. 1 is an elevational view, partly broken away, showing a plasma generator and accelerator embodying one form of the invention;

FIG. 2 is a fragmentary plan view, partly in section, of the apparatus shown in FIG. 1 taken along the line 2-2 of FIG. 1 and viewed in the direction of the appended arrows;

FIG. 3 is a fragmentary, perspective view of parts of the apparatus shown in FIG. 1;

FIG. 4 is an enlarged, fragmentary, sectional view of the plasma confinement vessel of FIG. 1 taken on the line 44 of FIG. 1 and showing the charged particles and their movements diagrammatically; and

FIGS. 5a and 5b are schematic diagrams of the circuits used in the apparatus shown in FIG. 1.

Referring more particularly to FIGS. 1 to 3, there is shown a vessel 10 having a reentrant base 12 which defines an annular trough 14 wherein a vaporizable material 15, such as mercury, may be located. The reentrant base 12 also defines a cylindrical tube 16 open externally at the bottom and closed at the top. An electric heater element 1 8 is inserted in the cylinder 16 and may be energized for heating and volatilizing the mercury to form a vapor or gas. This gas is introduced into a plasma confinement vessel 19 of a plasma generator and accelerator 20 through a conical inlet nozzle 22. The plasma confinement vessel 19 includes a cylindrical tube 24. The tube 24 is somewhat tapered or necked inwardly at one end which joins the nozzle 22. This tube 24 defines a chamber in which the plasma is generated and in which the energy of the electrons of the plasma is raised, as is explained hereinafter. The confinement vessel 19 also includes a conical outlet nozzle 26 joined to the tube 24. The flared outlet end of the nozzle 26 may be flanged. The vessel 10, the inlet nozzle 22, the tube 24, and the outlet nozzle 26 are desirably of a non-magnetic, insulating material, such as glass. Glass of the type sold under the trade name Pyrex or ceramics may be suitable.

A transmission line 30, including a pair of conductors 32, extends perpendicularly to the axis of the confinement vessel 19. Two plate electrodes 34 are individually connected by means of conductive stubs 35 to different ones of the conductors 32 of the line 30. The electrodes 34 are disposed diametrically opposite and facing each other adjacent the wall of the tube 24. The electrodes may be disposed internally of the tube, if desired. The conductors 32 may be connected to a transmission line 36, such as of the adjustable coaxial type. A radio frequency generator 37 may be connected to the coaxial line 36 (see FIG. 5b). A'shorting stub 39 is connected between the conductors 32 of the line 30 at a distance from the electrodes 34 equal to one-quarter wavelength of the radio frequency waves produced by the genera-tor 37. The coaxial line is connected to the transmission line 30 at a position between the electrodes 34 and stub 39 where the best impedance match is obtained. A radio frequency electric field may therefore be established between the plate electrodes 34. This field is transverse to the axis of the confinement vessel 19.

A pair of coils 40 and 42, which may be of the solenoidal type, are located around the inlet nozzle 22 and the outlet nozzle, respectively. The coils are coaxial with the axis of the confinement vessel 19 (i.e., the axis of the nozzles 22 and 26 and the cylindrical tube 24). The coils may be equidistant from the plate electrodes 34.

The plate electrodes 34 are parallel to the axis of the coils 40 and 42. The coil 40, around the inlet nozzle 22, may have a larger number of turns than the other coil 42 around the outlet nozzle 26. A structure 44 for mounting the coils may include a pair of flanges 46 and 48 spaced from each other by bolt-s 50 and associated nuts. The coils 40 and 42 may be cemented or strapped to their respective flanged cylinders 46 and 48. A cradle (not shown) may be used to support the coils 40 and 42 and their mounting structure in spaced relationship from the cylindrical tube 24 and the other elements of the plasma confinement vessel 19.

The coils 40 and 42 may be connected in series with a source of direct current, such as a battery 51 (see FIG. a). Each coil produces a solenoidal magnetic field in the same direction. The combined fields of both coils are illustrated by the dashed lines in FIG. 4. The field extends axially through the confinement vessel 19 and is essentially uniform and homogeneous within the tube 24. The field is pinched or concentrated (i.e., has a higher flux density) inwardly in the plane of the rearwardly located coil 40 so as to define a magnetic mirror to prevent escape of the plasma from the rear of the vessel 19. This mirror eifect occurs since the rearward coil 40 may have more turns than the forward coil 42 and therefore generates a stronger field. Alternatively, the coils 40 and 42 may be of equal numbers of turns and separately energized, the coil 40 with more current than the other coil 42.

When the accelerator is used for vehicle propulsion in free space (for example, over approximately 200 miles from the surface of the earth), the air pressure in the plasma confinement vessel 19 may be relatively low so that evacuation is unnecessary. For laboratory test purposes and where the plasma beam is to be used for purposes other than propulsion, the confinement vessel 19 is desirably evacuated, as by a vacuum pump.

A structure 54 is used for maintaining a vacuum in the confinment vessel 19 when desired. This structure includes a tube cross-joint 56 made of glass. Closure plates 58 seal the open upper and rear tube ends of the cross-joint 56. The front tube end of the cross-joint 56 is closed by a closure plate 60 having an aperture. The flanged end of the exhaust nozzle 26 is disposed against the closure plate 60 around the aperture thereof. A suitable ring gasket may be placed between the flanged end of the nozzle 26 and the plate 60. The lower tube end of the cross-joint 56 is located on a platform 62 having an opening 64 therein. This opening is in communication with a vacuum source, such as a pipe leading to a vacuum pump of the type known in the art. Measurement apparatus may be inserted through the upper and rear tube ends into the vacuum structure 54. For example, a Faraday cage 66 for ion measurement is attached to the rear closure plate 58.

The operation of the apparatus illustrated in FIGS. 1 to 3 may be best explained in connection with FIG. 4. This explanation is, in part, theoretical and is intended to provide 'a clearer understanding of the invention without restricting the invention to any particular mode of operation. The gas (for example, mercury vapor) enters the tube 24 through the nozzle 22. The gas may be ionized to form a plasma by the radio frequency electric field established by the electrodes 34, which field pene trates the chamber formed by the tube 24 into the plasma therein. The positive ions or neutral atoms comprising this plasma are represented by circles inscribed with a plus sign and the electrons are represented by a smaller circle inscribed with a minus sign. The particles of the plasma initially have a velocity component in a direction towards the outlet nozzle 26 since there is a pressure gradient in the tube 24 from the inlet nozzle 22 to the outlet nozzle 26. This pressure gradient is produced by the vacuum source (see FIG. 1). The ions and electrons come under the influence of the combined magnetic and radio frequency electric fields which are established in the chamber formed by the tube 24. The radio frequency electric field is preferably at the electron cyclotron resonance frequency of the plasma. For example, the radio frequency field may have a frequency of meg acycles per second for a magnetic field of: 50 gauss.

Consider a representative electron 70 in the plasma. This electron receives energy from the combined magnetic and electric fields and begins to gyrate. The radius of gyration becomes greater on each succesive orbit. Since the electron has an initial velocity in the axial directions towards the outlet nozzle 26, the electron has as slight corkscrew path of movement, which may be exaggerated in the drawing. After a number of gyrations depending upon the density of the plasma and the angular frequency of gyration of the electron, the electron may collide with a positive ion or a neutral atom and be deflected, for example, in the axial direction towards the outlet nozzle. In the illustrated collision of the electron 70 with a positive ion, some of the radial velocity of the electron is converted to axial velocity. The electron may continue to gyrate after the collision and pick up more and more energy from the electric field. Eventually, the electron travels out of the resonance chamber defined by the tube 24 into a second region in the exhaust nozzle 26. Desirably, but not necessarily, the concentration of particles is such that the electrons may execute on the average about one hundred gyrations before a collision. After a collision, the electron may have enough energy to travel out of the resonance chamber defined by the tube 24 without further interaction between the electron and the electric field, since the energy of the electron after collision may result in an electron velocity higher than the phase velocity of the electric field. The magnetic field extends in the axial direction into the exhaust nozzle. The electron 70 is confined by this magnetic field to drift in the axial direction.

Many collisions between ions and electrons occur within the tube 24. In some of these collisions, the radial velocity of the electrons is converted to velocity along the axial direction towards the inlet nozzle 22. Those electrons which enter the inlet nozzle 22 experience a magnetic force which deflects them back into the tube 24. This force is produced by the interaction of the moving electrons and the pinched magnetic field in the inlet nozzle region which establishes a magnetic mirror. The electrons reflected by the magnetic mirror reenter the tube 24 and again begin to gyrate.

The mobility of the electrons in a radially outward direction is much lower than the mobility of the electrons in the axial direction. The radial mobility of the electrons is limited by the axial magnetic field which interacts with the moving electrons and exerts a force thereon. After collisions, the magnetic field tends to redirect the electrons towards the common magnetic field and tube 24 axis. Many of the electrons therefore travel in the axial direction into the exhaust nozzle. The exhaust nozzle then contains a group of electrons having a very high energy. This group is enclosed by the dash line 74. Since the electrons are rapidly accelerated in the tube 24 and experience motion randomizing collisions, the temperature of the electrons is eflectively increased by the combined fields at electron-cyclotron resonance. The ions, on the other band, do not derive significant energy from the combined fields. Thus, the ions essentially remain at their initial temperature in the resonance chamber defined by the tube 24. The energetic electron gas expands. This expansion is in the axial direction due to the confining magnetic field. The expanding electron gas sets up space charge fields which electrostatically attract and accelerate the positive ions in the resonance chamber defined by the tube 24 out of that chamber into the exhaust nozzle 26. The acceleration of the ions by the space charge field occurs substantially instantaneously. The energy transfer from the electron group to the ions occurs without significant transfer of heat from the electrons to the ions by collision due to the high mass ratio of an ion to an electron. Thus, the energy transfer from the electrons to the ions is adiabatic and therefore very efiicient.

The electric and magnetic fields operate almost exclusively on the electrons and continuously accelerate electrons in the axial direction out of the resonance chamber defined by the tube 24. A plasma comprising a continuous flow of electrons and ions is continuously ejected from the nozzle 26. The thrust of the plasma beam is much greater than the thrust which might be produced by the magnetic and electric field forces acting upon the ions alone. Accordingly, the result of a high velocity plasma beam which is provided by the invention is unobvious from the field-ion interactions.

In an exemplary case, for a radio frequency field of approximately 100 volts per centimeter, a thrust of dynes per square centimeter may be measured on a mechanical vane suspended from the closure plate 58 (FIG. 1). The frequency of the radio frequency field in this exemplary case may be 140 megacycles per second and the magnetic flux density in the chamber 24 may be 50 gauss.

From the foregoing description, it will be apparent that there has been provided an improved method of and apparatus for charged particle and plasma acceleration. While a preferred embodiment of the accelerator is described above, other forms of accelerators embodying the invention (for example, with different electrode configurations for coupling R-F power into the plasma) will, no doubt, be apparent to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.

What is claimed is:

1. In apparatus for accelerating plasma located in a first region, a solenoidal magnetic field device positioned to provide a divergent field in a second region adjacent to and downstream from said first region, means for producing a mixture of electrons and positive ions in said first region, means for selectively driving the said electrons out of said mixture into said second region adjacent said first region to thereby establish electrostatic forces for attracting said positive ions into said second region and provide an accelerated plasma in said second region.

2. Apparatus for emitting a beam from a plasma including a mixture of positive ions and electrons comprismg (a) means for increasing the velocity of said electrons in said mixture without substantially increasing the velocity of said ions so that said electrons collide with said ions and are deflected,

(b) means for directing said deflected electrons out of said plasma in a predetermined direction into a re gion to thereby establish electrostatic forces which accelerate said ions in said predetermined direction to form a plasma beam, and

(c) a solenoidal magnetic field device positioned to provide a divergent field in said region.

3. Apparatus for providing a plasma beam including a mixture of electrons and positive ions comprising (a) means for generating a plasma in a first region, said plasma including said mixture of electrons and positive ions,

(b) means for selectively transferring energy to said electrons in said mixture for driving said electrons out of said mixture into a second region adjacent to and downstream from said first region to thereby establish electrostatic forces in said second region for attracting said positive ions into said second region and provide an accelerated plasma in said second region, and

(c) a solenoidal magnetic field device positioned to provide a divergent field in said second region.

4. Apparatus for generating and accelerating a plasma comprising (a) means for injecting a gas into a first region,

(b) means for ionizing said gas in said first region to provide a plasma including a mixture of electrons and positive ions,

(c) means for simultaneously and selectively driving said electrons out of said mixture into a second region adjacent to and downstream from said first region to thereby establish electrostatic forces for attracting said positive ions into said second region and thereby provide an accelerated plasma in said second region, and

(d) a solenoidal magnetic field device positioned to provide a divergent field in said second region.

5. Apparatus for accelerating a plasma located in a first region and including a mixture of electrons and positive ions comprising (a) means for establishing a pressure gradient in said plasma directed from said first region to a second region adjacent said first region, and

(b) means for applying electric and magnetic forces on said plasma in said first region for cyclically driving said electrons in their cyclotron resonance mode of gyration out of said first region into said second region for attracting said positive ions into said second region and thereby providing an accelerated plasma in said second region.

6. Apparatus for accelerating a plasma located in a first region and including a mixture of electrons and positive ions comprising (a) means for establishing a pressure gradient in said plasma directed from said first region to a second region adjacent said first region,

(b) means for applying to said plasma electric and magnetic fields thereby causing said electrons to gyrate in their electron cyclotron resonance mode of gyration into collisions with said ions in said first region, said electrons thereby being deflected into said second region, and

(c) means for confining said deflected electrons in said second region to form concentrated electrostatic forces for attracting said positive ions out of said first region into said second region to form a moving beam of plasma in said second region.

7. A plasma acceleration and generation apparatus comprising (a) means for injecting a gas into a chamber,

(b) means for establishing through said chamber a homogeneous magnetic field of a certain density, which field extends beyond one of two opposite ends of said chamber and is concentrated near the other of said opposite ends,

(c) means for establishing in said chamber a radio frequency electric field at the electron cyclotron resonance frequency for said magnetic field flux density thereby ionizing said gas into a plasma and accelerating said plasma toward said opposite ends of said chamber, and

(d) means for reflecting said plasma entering said other, opposite end back into said chamber.

8. A plasma accelerator comprising (a) means for establishing a plasma in a first region, said plasma including a mixture of electrons and positive ions,

(b) means for selectively driving said electrons out of said mixture into a second region downstream from and adjacent said first region for establishing electrostatic forces which attract said positive ions into said second region and providing an accelerated plasma in said second region, and (c) a solenoidal magnetic field device positioned to provide a divergent field in said second region.

9. Apparatus for generating a plasma beam comprising (a) means for establishing a plasma including a mixture of positive ions and electrons,

(b) means operative upon said plasma for separately and selectively increasing the velocity of said electrons in said mixture so that said electrons travel at high speed and collide with said ions and are deflected out of said mixture into a region,

(c) means for directing said deflected electrons in a predetermined direction away from said plasma for establishing electrostatic forces for accelerating said ions toward said electrons in said predetermined direction to form a beam, and

(d) a solenoidal magnetic field device positioned to provide a divergent field in said region.

10. A plasma accelerator comprising (a) a vessel,

(b) means for establishing in said vessel a plasma including a mixture of electrons and positive ions, and

() means for establishing magnetic fields in one direction through said vessel and electric and radio frequency electric fields in a direction transverse to said magnetic field in a predetermined portion of said vessel, said radio frequency electric fields selectively transferring energy to the electrons in said mixture and accelerating said electrons in the direction of said magnetic fields, said accelerated electrons establishing electrostatic forces for attracting said positive ions in the direction of said magnetic fields outwardly of said vessel.

11. A plasma generator and accelerator comprising (a) a vessel defining an interaction chamber,

(b) magnetic and radio frequency electric field generating means for establishing a magnetic field in said vessel which extends through said chamber and an electric field at electron cyclotron resonance frequency for said magnetic field in said chamber, and

(0) means for injecting a gas into said chamber for ionization in said chamber into a plasma which is accelerated out of said vessel.

12. A plasma propulsion device comprising (a) a vessel open at one end to provide an outlet and defining a chamber adapted to contain a plasma,

(b) a pair of electrodes disposed facing each other externally of said chamber,

(0) coil means for providing in said vessel a magnetic field of a certain flux density, which field is directed toward said outlet end, said field also being trans verse to a line between said electrodes, and

((1) means for applying to said electrodes radio frequency energy at the electron cyclotron resonance frequency for said magnetic field flux density in said chamber.

13. A plasma generator and accelerator comprising (a) a vessel defining a chamber and having inlet and outlet nozzles on opposite sides of said chamber, (b) a pair of coaxial solenoidal coils each disposed around said vessel,

(c) a pair of plate electrodes disposed facing each 8 other externally of said chamber between said coils, said electrodes being parallel to the axis of said coils,

(d) direct current means for energizing said coils to provide a concentrated magnetic field in said inlet nozzle, a homogeneous magnetic field of certain flux density in said chambers along said coil axis directed from said inlet to said outlet nozzle, and a field which extends into said outlet nozzle,

(e) means for applying radio frequency energy at the electron cyclotron resonance frequency for said certain magnetic field flux density in said chamber to said electrodes, and

(f) means for injecting a gas into said inlet nozzle for ionization in said chamber into a plasma which is nozzlef 14. A plasma propulsion device comprising (a) a tubular vessel defining a chamber and having inlet and outlet nozzles respectively at opposite ends of said chamber,

(b) a pair of solenoidal coils each disposed around said vessel and coaxial therewith,

(c) a pair of plate electrodes disposed facing each other externally of said chamber between said coils, said electrodes being parallel to the axis of said coils,

(d) means for energizing said coils to provide in said vessel an axial magnetic field of a certain flux density directed toward said outlet nozzle from said inlet to said outlet nozzle,

(e) means for reflecting a plasma entering said inlet nozzle into said chamber,

(f) means for applying radio frequency energy at the electron cyclotron resonance frequency for said certain magnetic field fiux density in said chamber to said electrodes, and

(g) means for injecting a gas into said inlet nozzle for ionization in said chamber into a plasma which is formed into a beam, accelerated and emitted from said vessel through said outlet nozzle.

References Cited by the Examiner UNITED STATES PATENTS 1/ 1949 Hergenrother 23069 7/1960 Blackman 31363 Aviation Week, Oct. 31, 1960, pp. 72, 74, 76, 77 and 79 relied on.

MARK NEWMAN, Primary Examiner.

SAMUEL LEVINE, Examiner. C. R. CROYLE, Assistant Examiner.

accelerated out of said vessel through said outlet 

1. IN APPARATUS FOR ACCELERATING PLASMA LOCATED IN A FIRST REGION, A SOLENOID MAGNETIC FIELD DEVICE POSITIONED TO PROVIDE A DIVERGENT FIELD IN A SECOND REGION ADJACENT TO AND DOWNSTREAM FROM SAID FIRST REGION, MEANS FOR PRODUCING A MIXTURE OF ELECTRONS AND POSITIVE IONS IN SAID FIRST REGION, MEANS FOR SELECTIVELY DRIVING THE SAID ELECTRONS OUT OF SAID MIXTURE INTO SAID SECOND REGION ADJACENT SAID FIRST REGION TO THEREBY ESTABLISH ELECTROSTATIC FORCES FOR ATTRACTING SAID POSITIVE IONS INTO SAID SECOND REGION AND PROVIDE AN ACCELERATED PLASMA IN SAID SECOND REGION. 